1. Theory

Definition: In the interdigitated back-contact (IBC) cell both contacts are on the rear side, as alternating stripes of an emitter and a base (back-surface-field) region with two metal fingers that interlock. The front has only a passivation and anti-reflection layer, so no metal shades the light. The junction is at the rear, which means that the carriers generated near the front must diffuse through the whole wafer to reach it, and the cell needs a high bulk lifetime and a very good front passivation.
Diffused version: Franklin et al. (ANU) built the cell on an n-type Czochralski wafer of 230 µm and 1.5 $Ω$ cm with a rear pitch of 500 µm: a boron emitter 330 µm wide (66 % of the area, 166 $Ω/ sq$ ), a localized phosphorus diffusion of 27 µm dots on a 70 µm spacing, an undiffused surface passivated by about 30 nm of thermal oxide and 80 to 100 nm of silicon nitride, aluminium fingers and contact dots of 7 µm. The front has silicon nitride with no diffusion, with $J_{0}$ of 5 fA/cm $^{2}$ . They measured 24.4 ± 0.7 % (703 mV, 41.95 mA/cm $^{2}$ , 82.7 %) on 4 cm $^{2}$ at Fraunhofer ISE; imec reported 23.3 % on 2 cm × 2 cm cells (696 mV) in 2011.
POLO versions: ISFH uses passivating contacts made of oxide and doped poly-silicon. The cell with an n-type POLO contact and a p-type POLO contact, separated by intrinsic poly-silicon, reached 26.10 ± 0.31 % (726.6 mV, 42.62 mA/cm $^{2}$ , 84.28 %) on 4 cm $^{2}$ , a record for p-type wafers. The industrial POLO IBC has a p-type Czochralski wafer, an aluminium base contact and a local n-type poly-silicon emitter deposited by PECVD through a shadow mask; ISFH reached 24.5 % on M2 wafers and expects less than 25.5 % because of recombination at the aluminium contact. The POLO2 version replaces the aluminium contact by a p-type poly-silicon stack with $J_{0}$ of 4 ± 1 fA/cm $^{2}$ and reached 25.5 % on a small area, with an implied voltage of 735 mV on cells without metal.
Heterojunction version: The same layout with amorphous-silicon contacts (page 80) is called HBC or HIBC. Kaneka, Panasonic and Sharp reported 26.3 %, 25.6 % and 25.09 %; LONGi reported 27.30 % in 2024 and 27.81 % in April 2025, certified by ISFH.
Modules: LONGi lists the Hi-MO 9 (HPBC 2.0) at 670 W and 24.8 % and an HIBC module with more than 700 W and up to 25.9 % on 2382 × 1134 mm; the ranking of TaiyangNews in May 2024 had AIKO ABC at 24.2 % and Maxeon at 24.1 % among modules. SunPower said in 2017 that its X-series cells exceeded 25 % in mass production.
Claims of the vendors: Rear contacts remove 3 to 5 % shading and the front ribbons, give a uniform appearance and, in a vendor blog, need two to four patterning steps more than TOPCon; none of these numbers is independently sourced.

2. Working Principle

Collection through the base: The holes generated near the front diffuse through the wafer to the rear emitter, so the diffusion length must be much larger than the wafer thickness plus half the pitch. For a lifetime of 5 ms the diffusion length is about 2.5 mm against 0.48 mm needed (computed, assumed diffusion coefficient). Franklin et al. state that a cell above 25 % needs a bulk lifetime above 10 ms and a front $J_{0}$ below 10 fA/cm $^{2}$ .
Front surface: With no diffusion the front depends only on the passivation layer; 5 fA/cm $^{2}$ corresponds to an equivalent velocity of 1.3 cm/s (computed).
Rear layout: Because the holes are the minority carriers, the emitter takes most of the rear (66 % in the ANU cell), and the base contact is small. The undiffused surface (32 %) has the lowest $J_{0}$ (21 fA/cm $^{2}$ ) and the contacts the highest (280 and 810 fA/cm $^{2}$ ), so the contacts are made as small dots of 0.13 % and 0.43 % of the area and the contact resistance is set by the dots (about 11 m $Ω$ cm $^{2}$ for the assumed resistivities). The design rules of the paper keep the metallised n+ below 0.2 % and the metallised p+ below 0.75 %.
Voltage: The saturation current of the rear is the area-weighted sum (37.6 fA/cm $^{2}$ , computed), and the rest of the 55 fA/cm $^{2}$ implied by 703 mV is the front and the bulk. Metal contact adds only 1.7 mV in this cell (computed).
Isolation of the polarities: In the POLO cell the p+ and n+ fingers are separated by a gap of intrinsic poly-silicon of 30 µm, and dopant interdiffusion widens the junction by a few µm (Rienaecker et al.). The hydrogenation of the rear stack lowers $J_{0}$ of the poly-silicon from 4 and 5.5 to below 0.5 and 3.3 fA/cm $^{2}$ for n+ and p+ in test structures.
Series resistance: The current runs through the fingers, which have 6.3 m $Ω$ /sq, and the rear metal can be thick because it does not shade (my reading); the resistive loss of the ANU cell is about 1 point, mostly in the base.

3. Applications

Premium modules: Maxeon and LONGi sell rear-contact modules for rooftops and for small areas where power density counts (248 to 259 W/m $^{2}$ for the LONGi examples, computed), and AIKO sells ABC modules. Maxeon cells are also quoted for solar race cars (vendor page).
Thin and large wafers: The rear junction suits thin wafers if the lifetime is high; the vendor blog gives 130 to 150 µm wafers of 1 to 5 $Ω$ cm.
Upgrade of PERC lines: ISFH changes only a few steps of a PERC+ process (doping steps), and Kalyon PV transferred it to an M10 line with an implied voltage of 727 mV.
Records: The HIBC cell (27.81 %) and the POLO IBC cell (26.1 %) are the highest silicon cell efficiencies reported in these pages, and the POLO2 potential is 26.4 %.

Rear Geometry and Recombination

Name	Formula	Related Formulae	Variables/Constants
Rear geometry and area fractions	$f_{i} = \frac{w _{i}}{p}$	$f_{d o t} = \frac{π d ^{2}}{4 p _{x} p _{y}}$ $f_{u n d i f f u se d} = 1 - f_{e} - f_{b}$	$f_{i} : a r e a f r a c t i o n o f a s t r i p e$ $w_{i} : i t s w i d t h$ $p : p i t c h o f o n e e mi tt er - an d - ba se p ai r$ $d : d iam e t er o f a d o t$ $p_{x}, p_{y} : s p a c in g o f t h e d o t s a l o n g an d a cr oss t h e f in g er s$ $f_{e}, f_{b} : f r a c t i o n s o f t h e e mi tt er an d o f t h e ba se d i f f u s i o n, F r ank l in e t a l . g i v e t h e w i d t h s an d t h e f r a c t i o n s b u t n o t t h ese f or m u l a s$ $(F r ank l in e t a l . : p = 500 μ m, e mi tt er 330 μ m (66 %), n f in g er 134 μ m (26.8 %), p f in g er 282 μ m (56.4 %), g a p 500 - 330 - 134 = 36 μ m an d u n d i f f u se d a r e a 32.1 % f or 1.9 % o f ba se d i f f u s i o n; d o t s o f 27 μ m (d i f f u s i o n) an d 7 μ m (co n t a c t) o n 70 μ m in o n e r o w p er 500 μ m g i v e 1.64 an d 0.11 % a g ain s t 1.9 an d 0.13 % p r in t e d, w hi c h cor r es p o n d t o a ce l l o f 70 \times 430 μ m; a s q u a r e a r r a y w o u l d g i v e 11.7 an d 0.79 %, co m p u t e d)$
Saturation current budget of the rear and the voltage	$J_{0, r e a r} = \sum_{i} f_{i} J_{0, i}$	$V_{oc} = \frac{k _{B} T}{q} ln (\frac{J _{sc}}{J _{0, f r o n t} + J _{0, r e a r} + J _{0, b u l k}} + 1)$	$J_{0, i} : s a t u r a t i o n c u r r e n t d e n s i t y o f t h e r e g i o n i (e mi tt er, ba se d i f f u s i o n, co n t a c t s, u n d i f f u se d s u r f a ce)$ $f_{i} : i t s a r e a f r a c t i o n$ $J_{0, f r o n t} : f r o n t s u r f a ce$ $J_{0, b u l k} : b u l k$ $J_{sc} : s h or t - c i r c u i t c u r r e n t d e n s i t y, t h e a r e a - w e i g h t e d s u m i s t h e s am e a s o n t h e A l - B S F an d T O P C o n p a g es$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d f r o m F r ank l in e t a l . : 0.43 % \times 810 (p co n t a c t) + 65.57 % \times 36 (e mi tt er) + 0.13 % \times 280 (n co n t a c t) + 1.77 % \times 190 (ba se d i f f u s i o n) + 32.1 % \times 21 (u n d i f f u se d) = 37.6 f A / c m^{2}; w i t h 5 f A / c m^{2} a t t h e f r o n t 42.6 f A / c m^{2} an d 709.6 mV a t 41.95 m A / c m^{2}, a g ain s t 703 mV m e a s u r e d, w hi c h im pl i es 54.9 f A / c m^{2}, so ab o u t 12 f A / c m^{2} r e main f or t h e b u l k an d t h e e d g e)$
Front surface saturation current and equivalent velocity	$J_{0, f r o n t} = \frac{q n _{i}^{2} S _{f r o n t}}{N _{D}}$	$S_{e q} = \frac{J _{0, f r o n t} N _{D}}{q n _{i}^{2}}$	$J_{0, f r o n t} : s a t u r a t i o n c u r r e n t d e n s i t y o f t h e f r o n t s u r f a ce$ $S_{f r o n t} : r eco mbina t i o n v e l oc i t y$ $N_{D} : d o n or co n ce n t r a t i o n o f t h e n - t y p e ba se$ $n_{i} : in t r in s i c co n ce n t r a t i o n, t h e r e l a t i o n i s t h e l o w - inj ec t i o n f or m f or a t hi c k ba se (ba c k g r o u n d k n o w l e d g e), F r ank l in e t a l . s t a t e t h e f r o n t J_{0} an d t ha t a ce l l ab o v e 25 % n ee d s l ess t han 10 f A / c m^{2} an d a b u l k l i f e t im e ab o v e 10 m s (c i t in g C o u s in s e t a l .)$ $(co m p u t e d a t 25 C w i t h n_{i} = 8.70 \times 1 0^{9} c m^{- 3} an d N_{D} = 3.1 \times 1 0^{15} c m^{- 3} (1.5 Ω c m, m o bi l i t y 1350 c m^{2} / V s a ss u m e d) : q n_{i}^{2} / N_{D} = 3.9 f A / c m^{2} p er c m / s, so 5 f A / c m^{2} i s 1.3 c m / s an d 10 f A / c m^{2} i s 2.5 c m / s)$
Collection of the minority carriers through the base	$L = D τ$	$L ≫ W + \frac{p}{2}$	$L : d i f f u s i o n l e n g t h o f t h e min or i t y h o l es$ $D : d i f f u s i o n coe f f i c i e n t$ $τ : b u l k l i f e t im e$ $W : w a f er t hi c k n ess$ $p : r e a r p i t c h, t h e co n d i t i o n i s m y es t ima t e o f t h e l o n g es t p a t h f r o m t h e f r o n t t o t h e ce n t r e o f a r e a r p i t c h, t h e so u r ces s t a t e o n l y t ha t t h e l i f e t im e m u s t b e hi g h$ $(co m p u t e d w i t h D = 12 c m^{2} / s (a ss u m e d) an d t h e m o d e l l e d l i f e t im e o f 5 m s o f F r ank l in e t a l . : L = 2450 μ m (1900 μ m f or 3 m s, 3460 μ m f or 10 m s), a g ain s t W + p /2 = 230 + 250 = 480 μ m)$

Contacts and Resistance

Name	Formula	Related Formulae	Variables/Constants
Contact resistance from the metal fraction	$R_{c} = \frac{ρ _{c}}{f _{c}}$	$R_{c, t o t a l} = R_{c, n} + R_{c, p}$	$R_{c} : r es i s t an ce o f t h e co n t a c t s o f o n e p o l a r i t y p er u ni t ce l l a r e a$ $ρ_{c} : s p ec i f i c co n t a c t r es i s t i v i t y$ $f_{c} : co n t a c t a r e a f r a c t i o n, F r ank l in e t a l . p r in t t h e r es i s t i v i t i es a s 1 \times 1 0^{5} an d 1.5 \times 1 0^{5} Ω c m^{2} (an d 2 \times 1 0^{5}, 7 \times 1 0^{5} in t h e d es i g n t ab l e), w hi c h I r e a d a s 1 0^{- 5} an d 1.5 \times 1 0^{- 5} w i t h t h e s i g n l os t, t h e r e l a t i o n i s s t an d a r d$ $(co m p u t e d w i t h 1 0^{- 5} an d 1.5 \times 1 0^{- 5} Ω c m^{2} (a ss u m e d r e a d in g) : n co n t a c t 1.0 \times 1 0^{- 5} /0.13 % = 7.7 m Ω c m^{2} an d p co n t a c t 1.5 \times 1 0^{- 5} /0.43 % = 3.5 m Ω c m^{2}, t o g e t h er 11 m Ω c m^{2}; w i t h t h e l i t er a l 1 0^{5} t h e v a l u e w o u l d b e ab o u t 1 0^{8} Ω c m^{2})$
Resistive loss in the rear fingers	$\frac{Δ P}{P} = \frac{J _{m pp} R _{s h} L ^{2}}{3 V _{m pp}} \frac{p}{w}$	$R_{f in g er} = \frac{R _{s h} L}{w}$	$Δ P / P : r e l a t i v e p o w er l oss in t h e f in g er s$ $J_{m pp}, V_{m pp} : c u r r e n t d e n s i t y an d v o l t a g e a t ma x im u m p o w er$ $R_{s h} : s h ee t r es i s t an ce o f t h e f in g er m e t a l$ $L : f in g er l e n g t h$ $p : p i t c h f r o m w hi c h o n e f in g er co l l ec t s$ $w : f in g er w i d t h, t h e e x p r ess i o n i s t h e s t an d a r d r es u l t f or a f in g er co l l ec t in g u ni f or m l y a l o n g i t s l e n g t h (ba c k g r o u n d k n o w l e d g e), F r ank l in e t a l . g i v e R_{s h} = 6.3 m Ω/ s q an d t h e w i d t h s 134 an d 282 μ m$ $(co m p u t e d w i t h L = 2 c m, J_{m pp} = 40 m A / c m^{2} an d V_{m pp} = 0.62 V (a l l a ss u m e d), p = 500 μ m : n f in g er 0.20 % an d p f in g er 0.10 %; f or L = 1 c m 0.05 an d 0.02 %; F r ank l in e t a l . a tt r ib u t e ab o u t 1 p o in t o f r es i s t i v e l oss m os tl y t o t r an s p or t in t h e b u l k)$
Metal-induced recombination and voltage loss	$Δ J_{0} = \sum_{m} f_{m} (J_{0, m e t, m} - J_{0, p a ss, m})$	$Δ V_{oc} = \frac{k _{B} T}{q} ln \frac{J _{0}}{J _{0} - Δ J _{0}}$	$f_{m} : f r a c t i o n o f t h e m e t a l co n t a c t$ $J_{0, m e t} : s a t u r a t i o n c u r r e n t d e n s i t y u n d er t h e co n t a c t$ $J_{0, p a ss} : v a l u e o f t h e p a ss i v a t e d r e g i o n o f t h e s am e d o p in g$ $J_{0} : t o t a l s a t u r a t i o n c u r r e n t d e n s i t y, t h e a r e a - w e i g h t e d f or m i s t h e s am e a s o n t h e T O P C o n p a g e, t h e J_{0} v a l u es a r e f r o m F r ank l in e t a l .$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : p co n t a c t 0.43 % \times (810 - 36) = 3.33 an d n co n t a c t 0.13 % \times (280 - 190) = 0.12, t o g e t h er 3.45 f A / c m^{2}, w i t h t h e im pl i e d t o t a l o f 54.9 f A / c m^{2} t hi s i s 1.7 mV; t h e d es i g n t ab l e o f t h e p a p er a s k s f or m e t a l l i se d n + b e l o w 0.2 % an d m e t a l l i se d p + b e l o w 0.75 %)$

Efficiency, Losses and Modules

Name	Formula	Related Formulae	Variables/Constants
Efficiency from the reported parameters	$η = \frac{V _{oc} J _{sc} F F}{P _{in}}, P_{in} = 100 mW/c m^{2}$	$η_{c h ec k} = \frac{V _{oc} J _{sc} F F}{1000}$ $η_{p o t} = \frac{i V _{oc} J _{sc} F F}{1000}$	$η : e f f i c i e n cy, V_{oc} in mV, J_{sc} in m A / c m^{2}, F F a s a f r a c t i o n (η_{c h ec k} i s in p er ce n t : t h e 1000 t u r n s mV in t o V, an d 1 mW / c m^{2} i s 1 p er ce n t o f P_{in})$ $η_{p o t} : p o t e n t ia l c a l c u l a t e d f r o m an im pl i e d v o l t a g e i V_{oc} an d an a ss u m e d f i l l f a c t or$ $(co m p u t e d : A N U 703, 41.95, 82.7 % g i v e 24.39 % (24.4 \pm 0.7); w i t h l a ser co n t a c t s 696, 41.4, 81.7 % g i v e 23.54 % (23.5); a l l - l a ser - d o p e d 671, 41.7, 68 % g i v e 19.03 % (19.1); m o d e l l e d 701, 42.1, 83.1 % g i v e 24.52 % (24.5); im ec 696, 41.6, 80.4 % g i v e 23.28 % (23.3); I S F H 726.6, 42.62, 84.28 % g i v e 26.10 % (26.10 \pm 0.31); P O L O 2 i V_{oc} = 735 mV w i t h 42.6 m A / c m^{2} an d an a ss u m e d 84.3 % g i v e 26.40 %, t h e p o t e n t ia l o f 26.4 % q u o t e d b y I S F H)$
Photocurrent budget	$J_{sc} = J_{g e n} - \sum_{i} Δ J_{i}$	$Δ J_{i} = q \int (Φ_{l os t, i}) d λ$	$J_{g e n} : p h o t o g e n er a t e d c u r r e n t d e n s i t y$ $Δ J_{i} : c u r r e n t s l os t b y r e f l ec t i o n, ab sor pt i o n in t h e co a t in g, in co m pl e t e l i g h t t r a pp in g an d f r ee - c a r r i er ab sor pt i o n$ $Φ : p h o t o n f l ux l os t, F r ank l in e t a l . l i s t t h e l osses b u t d o n o t s a y w h e t h er t h e i r p h o t o g e n er a t i o n c u r r e n t a l r e a d y e x c l u d es t h e r e f l ec t i o n$ $(F r ank l in e t a l . : 42.76 m A / c m^{2} min u s < 0.06 (co a t in g), 0.8 (l i g h t t r a pp in g) an d 0.05 (f r ee c a r r i er s) g i v es 41.85, a g ain s t 41.95 m e a s u r e d an d 42.1 m o d e l l e d; w i t h t h e r e f l ec t i o n o f 0.58 i t w o u l d b e 41.27, co m p u t e d)$
Efficiency loss budget	$η = η_{l im} - \sum_{i} Δ η_{i}$	$Δ η_{u na cco u n t e d} = η_{l im} - η - \sum_{i} Δ η_{i}$	$η_{l im} : l imi t o f t h e ce l l w i t h i d e a l o pt i cs an d o n l y A ug er an d r a d ia t i v e r eco mbina t i o n$ $Δ η_{i} : l osses in p er ce n t a g e p o in t s (o pt i c a l, r es i s t i v e, r eco mbina t i o n, e d g e), F r ank l in e t a l . g i v e 28.45 % f or t h e l imi t an d tw o se t s o f l osses$ $(F r ank l in e t a l . S ec t i o n 5 : 0.86 + 0.99 + 1.22 + 0.24 = 3.31 p o in t s; co n c l u s i o n s : 1.00 + 0.87 + 1.23 = 3.10 (3.34 w i t h t h e e d g e); 28.45 - 24.4 = 4.05 an d 28.45 - 24.5 = 3.95, so 0.64 t o 0.74 p o in t s a r e n o t a cco u n t e d f or in t h e f i r s t se t, co m p u t e d)$
Module efficiency from power and area	$η_{m o d} = \frac{P _{ma x}}{G A}$	$ρ_{P} = \frac{P _{ma x}}{A}$	$η_{m o d} : m o d u l e e f f i c i e n cy$ $P_{ma x} : ma x im u m p o w er$ $G : i r r a d ian ce o f 1000 W / m^{2}$ $A : m o d u l e a r e a in c l u d in g t h e f r am e e d g e o f t h e d a t a s h ee t$ $ρ_{P} : p o w er d e n s i t y, L O N G i g i v es 670 W an d 24.8 % f or t h e H i - M O 9 an d ab o v e 700 W, u p t o 25.9 % an d 259 W / m^{2} f or t h e H I B C m o d u l e o n 2382 \times 1134 mm$ $(co m p u t e d : t h e a r e a i s 2.701 m^{2}; 670 W g i v es 24.80 % an d 248 W / m^{2}; 700 W g i v es 25.91 % an d 259 W / m^{2})$
Gain from the absence of front metal	$\frac{Δ J _{sc}}{J _{sc}} = \frac{f _{s}}{1 - f _{s}}$	$J_{sc, I B C} = \frac{J _{sc, f r o n t - co n t a c t}}{1 - f _{s}}$	$f_{s} : f r a c t i o n o f t h e f r o n t s u r f a ce s ha d e d b y f in g er s an d b u s ba r s$ $J_{sc, I B C} : c u r r e n t d e n s i t y o f t h e ce l l w i t h o u t f r o n t m e t a l f or o t h er w i se e q u a l o pt i cs, t h e r e l a t i o n i s m y f i r s t - or d er es t ima t e, S u r g e P V (v e n d or b l o g) g i v es a s ha d in g o f 3 t o 5 % f or T O P C o n - t y p e ce l l s an d 2 t o 3 % f or m u l t i - b u s ba r d es i g n s$ $(co m p u t e d : f_{s} = 2, 3 an d 5 % g i v e + 2.0, + 3.1 an d + 5.3 % in c u r r e n t; t h e b l o g s t a t es a p o w er g ain o f 5 t o 7 % f or t h e w h o l e ce l l (u n so u r ce d))$

Important Questions

Ques: Is the back-contact heterojunction record 27.09, 27.30 or 27.81 %?

The sources give 27.09 % (November 2023), 27.30 % (May 2024, ISFH) and 27.81 % (April 2025, certified by ISFH, article of Solar Power World), all for LONGi cells. The steps are 0.21 and 0.51 points (computed). They are successive records, not a conflict: 27.81 % is the latest. SurgePV still calls 27.3 % the world record and labels it HBC in the text and HIBC in a table, which I take as an out-of-date claim and loose naming; the two names mean the same back-contact heterojunction layout. The Solar Power World article gives no voltage, current, fill factor or area for the 27.81 % cell, so the product check cannot be made. LONGi’s own cell-level data should be read from the certificate. Earlier 26.3 and 26.63 % belong to Kaneka. Reference: Solar Power World, ‘LONGi heterojunction back-contact solar cell reaches 27.81% efficiency’ (14 April 2025); SurgePV, ‘IBC solar cells explained’ (vendor blog); Silicon Heterojunction (HIT) Solar Cell page of this vault.

Ques: Is the efficiency limit of the IBC cell 28.45, 28.7 or 29.4 %?

Franklin et al. give 28.45 % for their cell with ideal optics and only Auger and radiative recombination, and 28.7 % for the Richter limit of a 230 µm wafer of 1.5 $Ω$ cm. The TOPCon page uses 29.4 % for silicon (Niewelt et al.), and ISFH says that the POLO IBC is limited to below 25.5 % by the aluminium contact and that the POLO2 cell has a potential of 26.4 %. They answer different questions: the first two come from a paper of 2014 to 2016 for one wafer, the third from a 2022 reassessment of the intrinsic recombination, and the last two are limits of one structure. The 29.4 % is more likely for the intrinsic limit today. I do not know which recombination model the 28.45 % uses, so the loss analysis of the paper is relative to a lower bound. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014); ISFH, ‘POLO IBC solar cells: How ISFH achieves industrial efficiencies of 24.5 percent and prepares the path to 26 percent’ (30 January 2026); TOPCon Silicon Cell page of this vault.

Ques: Which loss totals of the ANU cell are right?

Franklin et al. list in Section 5 an optical loss of 0.86 points, resistive 0.99, recombination 1.22 and edge 0.24, which add to 3.31 points. The conclusions give recombination 1.23, resistive 0.87 and optical 1.00, which add to 3.10, or 3.34 with the edge (computed). The difference between the limit and the cell is 4.05 points for 24.4 % and 3.95 for the modelled 24.5 %, so 0.64 to 0.74 points are not accounted for in the first set. The paper does not reconcile the sets. The recombination and the sum are close in both; the optical and resistive terms swap by 0.14 and 0.12 points. Section 5 is more likely the detailed result and the conclusions a rounded summary; the unaccounted part may be intrinsic terms, but the paper does not say. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014).

Ques: Is the contact resistivity of the ANU cell 1e5 or 1e-5 Ohm cm2?

Franklin et al. print the specific contact resistivities as 1 × 10 $^{5}$ $Ω$ cm $^{2}$ for the n contact and 1.5 × 10 $^{5}$ for the p contact, and 2 × 10 $^{5}$ and 7 × 10 $^{5}$ in the design table. A value of 10 $^{5}$ $Ω$ cm $^{2}$ with contacts of 0.13 % would give 10 $^{8}$ $Ω$ cm $^{2}$ , which is impossible for a cell with a fill factor of 82.7 %. Dropping the minus sign gives 7.7 and 3.5 m $Ω$ cm $^{2}$ for the contacts (computed), consistent with the series resistance. The extraction of the PDF is likely to have lost the superscript sign, so 10 $^{- 5}$ is more likely. I could not check the printed paper. If the true values were a decade larger, the contacts would cost about 1 point of fill factor, which the paper does not report. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014).

Ques: Does the photocurrent budget of the ANU cell add up?

Franklin et al. give a photogeneration current of 42.76 $mA/c m^{2}$ and lost currents of 0.58 for reflection, below 0.06 for the coating, 0.8 for incomplete light trapping and 0.05 for free carriers. Subtracting all four gives 41.27 $mA/c m^{2}$ , below the measured 41.95 and the modelled 42.1. Leaving out the reflection gives 41.85 (computed), within 0.1 of the measurement. The second reading is more likely: the 42.76 would be the current after the reflection. The paper’s extract does not say which, and the modelled 42.1 is 0.25 higher than 41.85, which may come from rounding of the small terms or from the model. The measured 41.95 is the number I would use for the product check, and the budget should be checked against the printed table of the paper. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014).

Ques: Are the area fractions of the dots 1.9 and 0.13 %?

Franklin et al. describe the base diffusion as dots of 27 µm diameter on a 70 µm pitch with a coverage of 1.9 %, and the contacts as dots of 7 µm on the same pitch with 0.13 % for the n and 0.43 % for the p contact. A square array on that pitch would cover 11.7 % and 0.79 %, and a single row at the finger pitch of 500 µm 1.64 % and 0.11 % (computed). The printed fractions correspond to one dot per 70 × 430 µm, close to the single row, so I read them as fractions of the whole cell with one row of dots per base finger. The extract does not give the layout. The printed values are more likely right; the difference of 15 to 18 % may come from the exact row spacing. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014).

Ques: Which saturation current does the p-type poly-silicon contact have?

Rienaecker et al. give a $J_{0}$ of 5.5 $fA/c m^{2}$ for the p+ contact of the precursor in one place and 10 in another, 3.3 after better hydrogenation, and 4 for the Al $_{2}$ O $_{3}$ reference stack; for the n+ contact they give 4, below 0.5 and 2.5 $fA/c m^{2}$ . Dullweber et al. report a median of 4 ± 1 $fA/c m^{2}$ for the SiO $_{x}$ and p-type poly-silicon stack of the POLO2 cell. The values describe different states of the stack, so they are not in conflict, but the 5.5 and 10 of the same precursor is an inconsistency inside the source. The 5.5 is more likely, since it agrees with the 4 ± 1 and the hydrogenation trend, whereas 10 is probably an earlier state or a typing error. For a finished cell 3 to 4 $fA/c m^{2}$ is the number to use. Reference: Rienaecker, Larionova, Kruegener, Wolter, Brendel and Peibst, ‘Rear side dielectrics on interdigitating p+-(i)-n+ back-contact solar cells: hydrogenation vs. charge effects’, EPJ Photovoltaics 12, 6 (2021); Dullweber et al., ‘Industrial implementation of 24%-efficient POLO IBC solar cells and future upgrade to 26%-efficient POLO2 IBC’, EPJ Photovoltaics 16, 30 (2025).

Ques: Which back-contact module has the highest efficiency: 24.1, 24.2, 24.8 or 25.9 %?

TaiyangNews lists AIKO ABC at 24.2 %, Maxeon at 24.1 % and LONGi Hi-MO 6 at 23.2 % (May 2024). LONGi states 24.8 % for the Hi-MO 9 (670 W) and up to 25.9 % for the HIBC module with more than 700 W, in 2025. The power and area give 24.80 % and 25.91 % (computed), so the numbers agree with the size of 2382 × 1134 mm. The values belong to different dates and products, and the 25.9 % is an ‘up to’ figure for a product described in a company statement. The listing is more likely to rank commercial modules at one date, and the 24.8 % is a rated value. The temperature coefficient is -0.24 %/°C for the HIBC (LONGi) against -0.26 to -0.29 for rear-contact cells in the vendor blog, which I treat as the same range. Reference: TaiyangNews, ‘Top 3 Efficiencies In H1 2024’; TaiyangNews, ‘LONGi Displays 25%+ Efficiency BC Module At SNEC 2025’ (1 July 2025); SurgePV, ‘IBC solar cells explained’ (vendor blog).

Ques: Is the base n-type or p-type, and how thick?

The ANU cell has an n-type Czochralski wafer of 230 µm (the design was for 200 µm) and 1.5 $Ω$ cm, imec used n-type float-zone wafers, and SunPower names n-type silicon. The ISFH POLO IBC cells are on p-type wafers (Czochralski, Ga-doped on M10, float-zone for POLO2) and the 26.1 % cell is a record for p-type material. The vendor blog gives 130 to 150 µm and 1 to 5 $Ω$ cm for n-type wafers. Both types work; n-type avoids boron-oxygen degradation, and p-type at ISFH keeps the process close to PERC lines (my reading). A thinner wafer helps the collection but loses current, so 130 to 230 µm is the range of the sources. The best choice depends on the lifetime, which the sources give only for the ANU model. Reference: Franklin, Fong, McIntosh, Fell, Blakers, Kho, Walter, Wang, Zin, Stocks, Wang, Grant, Wan, Yang, Zhang, Feng and Verlinden, ‘Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell’, Prog. Photovolt. 24(4), 411-427 (2016) (presented at the 29th EU PVSEC, 2014); imec press release on 23.3 % interdigitated back-contact cells (1 December 2011); ISFH, ‘POLO IBC solar cells: How ISFH achieves industrial efficiencies of 24.5 percent and prepares the path to 26 percent’ (30 January 2026); SurgePV, ‘IBC solar cells explained’ (vendor blog).

Ques: Is the voltage of the POLO cells 726, 727 or 735 mV?

ISFH reports Voc up to 726 mV for fully processed POLO IBC cells, the 26.1 % cell has 726.6 mV, an implied voltage of up to 727 mV was measured on M10 test wafers at Kalyon PV, and 735 mV on POLO2 structures without metal. These are different quantities: the first two are open-circuit voltages of finished cells, the last two implied voltages of wafers. At 42.6 $mA/c m^{2}$ they correspond to total saturation currents of 22.3, 21.9 and 16.1 $fA/c m^{2}$ (computed, single diode). The finished cell is lower than the implied value of its structure because of the metal and the edges, so I expect a POLO2 cell to be below 735 mV. The potential of 26.4 % assumes 735 mV and a fill factor of 84.3 % (computed), which is an upper estimate. Reference: ISFH, ‘POLO IBC solar cells: How ISFH achieves industrial efficiencies of 24.5 percent and prepares the path to 26 percent’ (30 January 2026); ISFH, ‘26.1 % record efficiency for p-type crystalline Si solar cells’ (web page); Dullweber et al., ‘Industrial implementation of 24%-efficient POLO IBC solar cells and future upgrade to 26%-efficient POLO2 IBC’, EPJ Photovoltaics 16, 30 (2025).

Quartz 5

Explorer

Interdigitated Back-Contact (IBC) Silicon Cell